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Abstract:

A method for forming a silicon film having a microcrystal structure is
provided. The method includes following steps. A plasma-enhanced chemical
vapor deposition system having a reaction chamber, a top electrode and a
bottom electrode is provided. The top electrode and the bottom electrode
are opposite and disposed in the reaction chamber. A substrate is
disposed on the bottom electrode. A silane gas is applied into the
reaction chamber. A silicon film having a microcrystal structure is
formed by simultaneously irradiating the silane gas in the reaction
chamber by a carbon dioxide laser and performing a plasma-enhanced
chemical vapor deposition step.

Claims:

1. A method for forming a silicon film, comprising: disposing a substrate
in an environment containing a silicon containing alkane gas; and forming
a silicon film on the substrate by simultaneously irradiating the silicon
containing alkane gas by a carbon dioxide laser and performing a
plasma-enhanced chemical vapor deposition.

2. The method for forming the silicon film according to claim 1, wherein
the silicon containing alkane gas comprises silane.

3. The method for forming the silicon film according to claim 1, wherein
the silicon film has a microcrystal structure.

4. The method for forming the silicon film according to claim 1, wherein
the silicon containing alkane gas comprises silane, the silicon film has
a microcrystal structure.

5. The method for forming the silicon film according to claim 1, wherein
a temperature of the plasma-enhanced chemical vapor deposition is
25.degree. C.-400.degree. C.

6. The method for forming the silicon film according to claim 1, wherein
a power of the plasma-enhanced chemical vapor deposition is 10 W-1000 W,
a pressure of the plasma-enhanced chemical vapor deposition is 0.1
torr-100 torr, a flow rate of the silicon containing alkane gas is 1
sccm-1500 sccm.

7. The method for forming the silicon film according to claim 1, wherein
a power of the laser is 10 W-1000 W.

8. A method for forming a silicon film having a microcrystal structure,
comprising: providing a plasma-enhanced chemical vapor deposition system,
wherein the plasma-enhanced chemical vapor deposition system comprises: a
reaction chamber; and a top electrode and a bottom electrode disposed in
the reaction chamber, wherein the top electrode is opposite to the bottom
electrode; disposing a substrate on the bottom electrode; supplying a
silane gas into reaction chamber; and forming a silicon film having a
microcrystal structure by simultaneously irradiating the silane gas in
the reaction chamber by a carbon dioxide laser by a power of 10 W-1000 W
and performing a plasma-enhanced chemical vapor deposition by a
temperature of 25.degree. C.-400.degree. C.

9. The method for forming the silicon film having the microcrystal
structure according to claim 8, wherein a power of the plasma-enhanced
chemical vapor deposition is 10 W-1000 W, a pressure of the
plasma-enhanced chemical vapor deposition is 0.1 torr-100 torr, a flow
rate of the silane gas is 1 sccm-1500 sccm.

Description:

[0001] This application claims the benefit of Taiwan application Serial
No. 99127170, filed Aug. 13, 2010, the subject matter of which is
incorporated herein by reference.

BACKGROUND

[0002] 1. Technical Field

[0003] The disclosure relates in general to a method for forming a silicon
film, and more particularly to a method for forming a silicon film having
a microcrystal structure.

[0004] 2. Description of the Related Art

[0005] A silicon material can be adjusted to have n-type conductivity or
p-type conductivity by a proper doing treatment. A p-n junction is
constructed by the n-type silicon and the p-type silicon in a silicon
solar cell. As the solar cell is irradiated by sunlight, an electron-hole
pair is generated by absorbing the photon by the junction depletion
region. The accumulating positive and negative charges are absorbed by an
electrode. After the electrode is connected with a load, an electrical
current is generated due to a potential difference. Therefore, the light
energy is transformed into the electrical energy by the solar cell.

[0006] The single crystal silicon is mainly used in the silicon solar cell
early. However, it is not easy to get the single crystal silicon and the
single crystal silicon is expensive. The trend of the material is
directed toward other substitutive materials, such as a recovered silicon
material or other non-single crystal materials.

[0007] However, a solar cell using an amorphous silicon film has a
photoelectric conversion lower than that of a solar cell using a single
crystal silicon or polycrystal silicon having an atomic arrangement order
in a long or short range better than that of the amorphous silicon film.
In addition, the stability of the amorphous silicon film solar cell is
limited due to Staebler-Wronski effect, that is, a long-term irradiation
results in degradation of the film quality and photoconductivity.

[0008] In 1974, it is found that the conductivity of the amorphous silicon
can be stabilized and the probability of the light-induced degradation
can be decreased by filling a hydrogen atom into a dangling bond of the
amorphous silicon for decreasing the recombination center of the deep
level and the recombination probability of the carrier.

[0009] Currently in some researches, the silicon material is transformed
into the crystal phase from the amorphous phase by certain methods. For
example, it is found the silicon material can be changed into the
microcrystal phase from the microcrystal structure phase by adjusting the
hydrogen flow rate. However, the hydrogen atom in the silicon material
must be moderate for preventing from a loose structure.

[0010] Z. Tang et al. (Z. Tang, W. Wang, B. Zhou, D. Wang, S. Peng, and D.
He, "The influence of H2/(H2+Ar) ratio on microstructure and
optoelectronic properties of microcrystalline silicon films deposited by
plasma-enhanced CVD", Appl. Surf. Sci., 255, 8867 (2009) use a
plasma-enhanced chemical vapor deposition system to deposit the amorphous
silicon film by parameters of a SiH4 flow rate: H2 flow rate of
30:100, a pressure of a reaction chamber of 800 mtorr, and a temperature
of a substrate of 400° C. Next, the formed amorphous silicon film
is transformed into a microcrystal structure by a high temperature of
550° C.-850° C. However, the high temperature process is
not suitable for the glass or polymer substrate not temperature resistant
material. The method for forming this kind of the silicon film having the
microcrystal structure could only applied a limited condition.

SUMMARY

[0011] A method for forming a silicon film is provided. The method
comprises following steps. A substrate is disposed in an environment
having a silicon containing alkane gas. A silicon film is formed on the
substrate by simultaneously irradiating the silicon containing alkane gas
by a carbon dioxide laser and performing a plasma-enhanced chemical vapor
deposition step.

[0012] A method for forming a silicon film having a microcrystal structure
is provided. The method comprises following steps. A plasma-enhanced
chemical vapor deposition system is provided. The plasma-enhanced
chemical vapor deposition system comprises a reaction chamber, a top
electrode and a bottom electrode. The top electrode and the bottom
electrode opposite to the top electrode are disposed in the reaction
chamber. The substrate is disposed on the bottom electrode. A silane gas
is supplied into the reaction chamber. A silicon film having a
microcrystal structure is formed by simultaneously irradiating the silane
gas by a carbon dioxide laser in the reaction chamber and performing a
plasma-enhanced chemical vapor deposition step.

[0016] In Embodiments of the application, a silicon film having a
microcrystal structure can be formed under a low temperature due to using
a plasma-enhanced chemical vapor deposition in combination with using a
carbon dioxide laser. The low temperature process used in embodiments
allows using various substrates not temperature resistant such as glass
or polymer, etc.

[0017] In embodiments of the application, a method for forming the silicon
film having the microcrystal structure comprises following steps. A
substrate is disposed in an environment containing a silicon-containing
alkane gas. Next, the silicon film is formed on the substrate by
simultaneously irradiating the silicon containing alkane gas by a carbon
dioxide laser and performing a plasma-enhanced chemical vapor deposition.

[0018] In embodiments of the application, the silicon containing alkane
gas comprises silane. The laser is carbon dioxide laser. For example, a
power of the laser is 10 W-1000 W.

[0019] In one embodiment, for example, a temperature of the
plasma-enhanced chemical vapor deposition is 25° C.-400° C.
A power of the plasma-enhanced chemical vapor deposition is 10 W-1000 W.
A pressure of the plasma-enhanced chemical vapor deposition is 0.1
torr-100 torr. A flow rate of the silicon containing alkane gas is 1
sccm-1500 sccm.

[0020] The silane gas has a high absorbing efficiency for a carbon dioxide
laser having a wavelength of about 10.6 μm, and thus a dissociation
rate of a Si--H bond of the silicon containing alkane gas is increased
due to the carbon dioxide laser. Therefore, in one embodiment, by using
the plasma-enhanced chemical vapor deposition system in combination with
the carbon dioxide laser, the silicon film having the microcrystal
structure can be formed by using a plasma power of a low frequency such
as 13.56 MHz in an environment at a low temperature (25°
C.-400° C.).

[0021] The advantages of the present disclosure are illustrated with the
following examples of the present disclosure and comparative examples.

[0022] <Reaction Device>

[0023] FIG. 1 and FIG. 2 illustrate a reaction device in embodiments. The
reaction device comprises a plasma-enhanced chemical vapor deposition
system 10 and a carbon dioxide laser system 20.

[0025] (1) A reaction chamber 11 has a diameter of about 40 cm. The
reaction chamber 11 has a transparent window 12 which may be formed by
glass, quartz or a transparent crystal chip, etc. A top electrode 31
(FIG. 2) and a bottom electrode 32 (FIG. 2) are disposed in the reaction
chamber 11. Diameters of the top electrode 31 and the bottom electrode 32
are about 25 cm. The top electrode 31 is opposite to the bottom electrode
32. A gap between the top electrode 31 and the bottom electrode 32 is
about 3 cm.

[0026] (2) A radio frequency of a radio frequency generator 17 is 13.65
MHz. A power of the radio frequency generator 17 matched with a LC can be
up to 300 W. In addition, an output power value and a reflection power
value can be read from a panel of the radio frequency generator 17
directly.

[0027] (3) A rotary pump 16 and a Roots pump 15 connected in series are
used in a vacuum system. The rotary pump 16 has an automatic pressure
controller for accurately controlling a pressure. The Roots pump 15 can
generate a huge amount of an air displacement, increasing a rate of
pumping the reaction chamber 11 to a vacuum state. A filter screen is
disposed at a suction port of the pump for preventing the pump from
defect due to a solid impurity.

[0028] (4) A master flow controller (MFC) 18 is used for accurately
controlling a flow rate of a silicon containing alkane gas flowing into
the reaction chamber 11 from a pipe 13. The pipe 13 also can be
communicated with high-purity nitrogen for purging the pipe 13 after the
reaction.

[0029] A laser wavelength of the carbon dioxide laser system 20 is 10.6
μm. A laser power of the carbon dioxide laser system 20 can be up to
100 W by using a laser controller 21. A path of the carbon dioxide laser
system 20 is adjustable.

[0030] <Silicon Film Deposition>

[0031] In a process for depositing the silicon film, the substrate 33 is
disposed on the bottom electrode 32 in the reaction chamber 11. Then,
simultaneously, a plasma-enhanced chemical vapor deposition step is
performed, and the silane (SiH4) gas in the reaction chamber 11 is
irradiated by the carbon dioxide laser through the transparent window 33
from the carbon dioxide laser system 20.

[0032] In the plasma-enhanced chemical vapor deposition step, the silane
gas flowing into the reaction chamber 11 has a concentration of 4% and a
flow rate of 250 sccm. An argon gas is used as a solvent for the silane
gas. The pressure in the reaction chamber 11 is controlled to be 0.5
torr. The radio frequency is 13.65 MHz. The power is 300 W. The
plasma-enhanced chemical vapor deposition step is performed in an
environment having a room temperature (about 25° C.) for about 30
minutes.

[0033] The path of the laser from the carbon dioxide laser system 20 is
controlled to cross above the substrate 33 and be as close to the
substrate 33 as possible, for maximizing the reactive gas close to a
surface of the substrate 33 to be irradiated by the laser and deposited
onto the substrate 33. In this embodiment, the laser is stopped on the
bottom electrode 32 at a left side of the substrate 33. An included angle
θ between the laser and the bottom electrode 32 is about 2 degrees.

[0036] FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D and FIG. 3E respectively are
Raman spectral figures of the silicon films formed by carbon dioxide
laser powers of 0 W, 23 W, 45 W, 68 W and 90 W. The curve A is a result
measured by a Raman spectrometer. The curve B and the curve C are fitted
curves from the cuve A. The curve B and the curve C respectively have
mode peaks at different positions of wavenumber.

[0037] Generally, in a result of the Raman spectrometer, an amorphous
silicon would have a wide mode peak at about 480 cm-1. A crystal
silicon would have a narrow mode peak at about 520 cm-1. As a
crystallization degree of the crystal silicon becomes more high, a full
width at half maximum (FWHM) of the mode peak is more narrow. Therefore,
the single crystal silicon has the narrowest FWHM. The FWHM of the mode
peak at 480 cm-1 is bigger than the FWHM of the mode peak at 520
cm-1. Thus, for example, as the structure of the film gradually
becomes the crystal phase from the amorphous phase, the position of the
mode peak would gradually shift to 520 cm-1 from 480 cm-1, and
the FWHM would be narrower. In addition, as a ratio of the amorphous
structure to the crystal structure of the film becomes smaller, a ratio
of the area of the mode peak at 480 cm-1 to the area of the mode
peak at 520 cm-1 would become smaller. Therefore, a relative
relation between the crystal silicon and the amorphous silicon of the
silicon film can be analyzed by the position, the FWHM and the area of
the mode peak of the Raman spectral curve.

[0038] In FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D and FIG. 3E, the position of
the mode peak of the curve B is at about 480.0 cm-1, indicating the
existence of the amorphous silicon. The position of the mode peak of the
curve C is at about 513.6 cm-1-518.4 cm-1, indicating the
existence of the crystal silicon. Table shows results obtained from FIG.
3A, FIG. 3B, FIG. 3C, FIG. 3D and FIG. 3E. The volume fraction Xc of
the crystal structure is obtained by dividing the area of the mode peak
of the curve C by a total area of the mode peak of the curve B and the
mode peak of the curve C. From the result of table 1, it is found that as
the power of the carbon dioxide laser becomes higher, a ratio of the
crystal silicon structure to the amorphous silicon structure of the
silicon film becomes higher.

[0040] FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D and FIG. 4E respectively shows
results of the silicon film formed by using carbon dioxide laser powers
of 0 W, 23 W, 45 W, 68 W and 90 W measured by the Fourier transform
infrared spectrophotometer. In figures, the curve D is the result
obtained by deducting the substrate signal from the infrared spectral
analysis. The curve E and the curve F are curves fitted from the curve D.
The curve E and the curve F respectively have mode peaks at different
positions of wavenumber.

[0041] In figures, the range of the absorption curve is 1900
cm-1-2200 cm-1, belonging to the signal of Si--H bond. The
position of 2100 cm-1 mainly belongs to SiH2 bond. A low
absorption intensity of the mode peak at 2100 cm-1 indicates a small
quantity of SiH2 bond, indicating a low probability of defect of the
silicon film structure. The position at 2000 cm-1 mainly belongs to
SiH bond. A high ratio of the absorption intensity of the mode peak at
2000 cm-1 to the absorption intensity of the mode peak at 2100
cm-1 indicates a small quantity of the defect and a nice quality of
the silicon film. On the contrary, a low ratio of the absorption
intensity of the mode peak at 2000 cm-1 to the absorption intensity
of the mode peak at 2100 cm-1 indicates a huge quantity of the
defect, a loose structure and a nasty quality of the silicon film.

[0042] From results shown in FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D and FIG.
4E, it is found that as the power of the carbon dioxide laser becomes
higher, a ratio of the intensity of the absorption peak at 2000 cm-1
(curve F) to the intensity of the absorption peak at 2100 cm-1
(curve E) becomes higher. It indicates that the silicon film has a low
amount of defect and high quality. Particularly, the silicon film in FIG.
4E has the best quality as the absorption intensity of the mode peak at
2100 cm-1 is almost zero.

[0043] <Transmission Electron Microscope (Tem) Test>

[0044] In the result of the Raman spectral test, the Raman signal shows
that the silicon film has the crystal structure. However, it is still
need to further analyze if the volume of the formed crystal structure is
big enough to be distinguished from the amorphous structure. The
structure of the silicon film is observed by a TEM herein. Situation of
the crystal structure formed by various laser powers are also discussed.

[0045] No crystal particle is observed and only an amorphous silicon film
of a big area is observed in the bright field image of the silicon film
formed without irradiating the silicon containing alkane gas by the laser
(in other words, the laser power is 0 W). Also, no signal of the crystal
silicon is found in the diffraction ring image of the silicon film formed
without irradiating the silicon containing alkane gas by the laser (in
other words, the laser power is 0 W), identical to the result of the
bright field image.

[0046] It is observed that in the bright field image, the silicon film
formed by irradiating the silicon containing alkane gas by the laser of
68 W has a microcrystal particle of an average diameter of about 2 nm.
The diffraction ring image of the silicon film formed by irradiating the
silicon containing alkane gas by the laser of 68 W also reveals a weak
diffraction ring pattern of a lattice plane.

[0047] It is observed that in the bright field image, the silicon film
formed by irradiating the silicon containing alkane gas by the laser of
90 W has a microcrystal particle of an average diameter of about 5 nm,
bigger than the microcrystal particle formed by irradiating by the laser
of 68 W. It is presumed the silicon crystal particle of big size is
formed because much crystal particles are formed due to a high laser
power and the probability of growth resulted from stacking of crystal
particles is high, or, the growth rate of the silicon crystal particle is
improved by increasing the laser power. The diffraction ring image of the
silicon film formed by irradiating the silicon containing alkane gas by
the laser of 90 W also reveals a diffraction ring pattern of strong
bright intensity of a lattice plane. The signals of the inner ring to the
outer ring, obtained by calculating the gap distance between the rings,
respectively represent the silicon crystal planes of (111), (220) and
(311).

[0048] In embodiments, the silicon film having the microcrystal structure
and excellent quality can by manufactured by using the plasma-enhanced
chemical vapor deposition in combination with the carbon dioxide laser at
the low temperature.

[0049] While the disclosure has been described by way of example and in
terms of the exemplary embodiment(s), it is to be understood that the
disclosure is not limited thereto. On the contrary, it is intended to
cover various modifications and similar arrangements and procedures, and
the scope of the appended claims therefore should be accorded the
broadest interpretation so as to encompass all such modifications and
similar arrangements and procedures.